Pilot valves of the block and bleed type commonly use a pressure balance spool valve which is shiftable, responsive to a change in a controlled sensed pressure to apply or exhaust a control pressure to a motor valve, thereby controlling the latter. Thus the valve element is normally urged to one position by a spring whose force is opposed by the fluid pressure to be controlled acting on a piston to urge the valve element towards a second position. Pressure sensing valves of this type generally incorporate as part of the valve element, or separate from the valve element and engaging the valve element, a piston surface within a piston chamber exposed to the sensing fluid pressure so that the area of the piston member determines the force at which a specified pressure will overcome the spring force opposing the fluid pressure to urge the valve element towards its second position.
For a specified piston area pressure surface and a specified spring, a pressure sensor pilot valve has a specified range of pressure to which the "trip" pressure may be adjusted. In order to accomodate different values of pressure ranges, pilot valves incorporated the use of number of springs of varying compression force and/or different piston and housing assemblies having varying piston surface areas such as is shown in U.S. Pat. No. 4,258,741 or my co-pending application entitled "Adjustable Pressure Sensing Piston Mechanism", Ser. No. 126,691 filed Mar. 3, 1980. This practice necessitated interchanging these various springs and/or piston and housing assemblies to accomodate a certain specific range of "trip" pressure in order for the sensing pilot valve to function properly.
Further, previous designs required adjustment mechanisms involving a large number of parts and the use of multiple seals such as O-rings within the piston apparatus.